While the dimension of Otherworld is also known as Avalon, the island of Avalon is actually only a small part of the large dimension that is Otherworld. Other places within this dimension include Fomor, home of the Fomorians, Annwn, home of the dead, and many other locales associated with Celtic mythology. The Starlight Citadel is home of Roma and the base of the Captain Britain Corps.

In general, Otherworld is a High fantasy world; gods, elves, dragons and many more live within small towns and large castles with no heavy industry. Large forests and bright lakes separate the several islands and locations from one another. Otherworld is not primitive though; while sorcery and magic are a part of daily life, the Captain Britain Corps and Avalon possess highly advanced technology.

In ancient times, there were many ways to travel from Earth to Avalon and back, created by powerful men known as the Twelve Walkers. Six of the Twelve Walkers became corrupt and created false paths that would lead travellers to dangerous places or insanity. The actions of these evil Walkers caused a backlash from the human inhabitants of Earth against all magical creatures and many of these creatures followed the paths to relocate to Otherworld. Most of the paths between Earth and Avalon were destroyed and only a few remain.

Merlyn created the Starlight Citadel, located on a different island than Avalon. From the Starlight Citadel he founded the Captain Britain Corps, a group of superhuman heroes who would patrol the countless Earths in the Omniverse. Merlyn became the Omniversal Guardian and was assisted in his task by the Omniversal Majestor/Majestrix. His task was to safeguard the Omniverse against any threats and the technology Merlyn provided was capable of destroying entire universes that were a threat. When Mad Jim Jaspers appeared, Merlyn faked his death and was replaced by Roma as Omniversal Guardian. Saturnyne became the Omniversal Majestrix.

Years later, Mastermind conquered the island of the Starlight Citadel, destroyed most of the Captain Britain Corps and disguised himself as Roma. Mastermind wanted the Amulet of Right and the Sword of Might, two mystical artifacts which had once empowered Brian Braddock, also known as Captain Britain and his enemy Joshua Stragg, the Reaver. Braddock found the artifacts before Mastermind, and discovered that he was the rightful heir to them. He used them to defeat Mastermind, and Roma named him the new ruler of Otherworld, while she remained as his advisor. Brian recently left his throne after the events in House of M and returned to Earth.

On the island of Avalon, home to the Celtic Gods also known as the Tuatha da Danaan, Druids would forge the magical artifact known as the Evil Eye. The Eye was given to Prester John, who travelled back to Earth with it. The Celtic Gods themselves would have an endless war with the Fomorians, who would try to invade Avalon repeatedly. The Celtic Gods would receive assistance from Thor in repelling several invasions.

One of the Celtic Gods, the Lady of the Lake safeguarded Excalibur, King Arthur's sword. She became a close ally of the Black Knight and even gave him Excalibur when he lost his weapon, the Ebony Sword. Years later, the Lady of the Lake gave the Black Knight his new weapons: the Sword of Light and the Shield of Night. In return the Black Knight became her protector. The Black Knight would often travel between Earth and Avalon and seek the Lady of the Lake for advice.

Avalon is also the home of the Green Knight, a primordial nature spirit. From the Green Chapel he empowers select warriors as his Pendragons to battle the servants of the Red Lord and his followers; known as the Bane. Pendragons of the past include Arthur, Merlin & the Knights of the Roundtable, Robin Hood and the Merry Men, the WWI superhero Albion, detective Dai Thomas, Captain Britain, Union Jack, and others.

The Lady of the Lake called Black Knight "The new Pendragon" when she gave him the Sword of Light and Shield of Night, it is unclear if Dane was tapping into the same mystical power that empowered the other Pendragon Knights.

The Green Knight's Pendragons support nature, creation, and life; while the Bane champion decay, destruction and death. The Red Lord operates from a hell-realm called Anwyn.

During the Secret Invasion storyline, the Skrulls invaded Avalon where they seemingly destroy the Lady of the Lake and the Green Knight.[1] Pete Wisdom later used the shard to free Merlin from his imprisonment who managed to revive Captain Britain.[2] When Merlin gave Captain Britain Excalibur, he used to end the Skrull's invasion of Britain. Pete Wisdom later restored the Lady of the Lake and the Green Knight to life.[3]

Captain Britain and Meggan: When Brian Braddock became ruler of Otherworld, he relocated together with Meggan to live on Otherworld. After Meggan's disappearance during House of M, Brian returned to his homeworld Earth-616.

Green Knight: Lord Bercilak de Hautdesert is the Green Knight of unknown origin.

The Lady of the Lake: A Celtic goddess who lives both on Camelot and on Earth at the same time.